Fast neutron nuclear reactors of integrated type comprise a main vessel containing liquid metal such as sodium, forming the liquid for cooling the nuclear reactor in which is immersed the reactor core consisting of fuel assemblies. The main vessel of the reactor is divided internally into two regions, by means of a complex structure forming the inner reactor vessel. This complex structure is equivalent to a wall, a part of which, known as a stepped wall, extends radially relative to the main vessel.
Of the two regions bounded by the inner vessel, the first is arranged substantially in the upper part of the vessel and the other in the lower part. The upper region, known as the hot header, communicates with the core outlet and receives the hot liquid metal which has passed through the core fuel assemblies. The lower region, known as the cold header, receives the sodium cooled in the intermediate exchangers immersed in the main reactor vessel. This cooled liquid metal is then conveyed from the cold header to the lower part of the core assemblies.
When a nuclear reactor has operated for a certain period of time, it continues to release significant residual power when it is stopped, i.e. when the reactor control rods are inserted into the core, in their maximum insertion position. The residual power of the reactor must therefore be removed to avoid damage to internal components and structures owing to an excessive temperature rise.
This possibility of removing the residual reactor power must be maintained even when the reactor has undergone major breakdowns and when the main power removal circuits, which are employed when the reactor runs normally, are out of action.
Emergency circuits are therefore resorted to, these being used only when the reactor is stopped and when the main circuits are out of action.
In the case of fast neutron nuclear reactors cooled by liquid sodium and of an integrated type, use is made of emergency heat exchangers immersed in the reactor vessel, inside the hot header. These emergency heat exchangers, in which the liquid sodium reactor coolant circulates, are associated with heat exchangers of the sodium-air type, arranged outside the reactor vessel and responsible for cooling the secondary liquid sodium which has been heated by coming into thermal contact with the primary sodium reactor coolant, passing through the exchanger immersed in the vessel. These emergency exchangers, associated with sodium-air exchangers, form circuits which are completely independent of the main circuits.
The emergency exchangers, which are immersed directly in the hot header of the vessel, comprise inlet openings for the sodium to be cooled in their upper part and outlet openings for the cooled sodium in their lower part. The cooled sodium is therefore reintroduced into the hot header and must follow a complicated path in order to travel into the cold header and, from there, to return to the lower part, or bottom, of the fuel assemblies. This complex path includes passing through pumps and the bodies of the intermediate exchangers of the main power removal circuits. This results in fairly large pressure drops, reduced efficiency of the reactor emergency cooling device and considerable temperature dissymmetries in the hot header, leading to additional thermal stresses on the stepped wall.